Muscle fibers are activated by motor neurons, which carry electrochemical signals from the spinal cord. Each motor neuron has multiple terminal branches, so it can innervate many muscle fibers. The motor neuron that innervates a fiber strongly influences that fiber’s type, characteristics, and role during exercise.

Activation of muscles

Muscle activation starts when a motor neuron sends an impulse from the spinal cord to the muscle fibers it innervates. A motor unit is one motor neuron plus all the muscle fibers it controls. Activation follows the all-or-nothing principle: when a motor unit is stimulated, all fibers in that motor unit contract together.

The strength and precision of muscle movement depend on:

• Motor unit size: Small motor units are recruited for fine, precise movements (e.g., in the eye), while large motor units are used for gross, powerful movements (e.g., in the quadriceps).
• Recruitment pattern: Force increases by recruiting more motor units or by increasing the firing frequency of already active motor units.

Motor unit recruitment follows the size principle: smaller, low-threshold Type I fibers are activated first, followed by larger, high-threshold Type II fibers as force demands increase.

Steps of muscle contraction

1. Energizing the myosin head: ATP is hydrolyzed by myosin ATPase, which energizes the myosin head.
2. Cross-bridge formation: Myosin heads attach to binding sites on actin.
3. Power stroke: Myosin pulls actin toward the center of the sarcomere, shortening the muscle. ADP is released.
4. Detachment: A new ATP molecule binds to myosin, causing myosin to detach from actin.
5. Reset: Myosin returns to its original position, ready to form another cross-bridge if calcium and ATP are available.

This cycle continues as long as calcium ions (released from the sarcoplasmic reticulum) remain bound to troponin. Calcium binding shifts tropomyosin and exposes actin’s binding sites. When calcium is removed and ATP is available, the muscle relaxes. ATP depletion prevents normal detachment and contributes to loss of contraction control.

Muscle fiber types

Skeletal muscles are composed of two primary fiber types:

1. Type I (Slow-twitch):
   • High aerobic capacity and resistance to fatigue.
   • Used in activities like distance running.
2. Type II (Fast-twitch):
   • Subdivided into Type IIa and IIx fibers.
   • Generate rapid, powerful contractions but fatigue quickly.
   • Ideal for activities like sprinting and weightlifting.
   • With consistent training, Type IIx fibers can shift toward Type IIa characteristics.

Fiber type	Characteristic	Best suited for
Type I	High endurance, low force output	Long-distance running
Type IIa	Intermediate endurance and force	Mid-distance running
Type IIx	Low endurance, high force output	Sprinting, powerlifting

Force production in muscle

Muscle force depends on how many cross-bridges form between actin and myosin at a given time.

• Key factors:
  • Motor unit recruitment: Activating more motor units increases force.
  • Rate coding: A higher frequency of neural signals produces summation and greater force output.
  • Motor unit synchronization: More coordinated firing of motor units can increase the efficiency and magnitude of force production.
  • Muscle cross-sectional area: Larger muscles contain more contractile proteins, which allows greater force generation.

Characteristics of muscle fiber types

This table summarizes the main features of muscle fiber types and their functional properties:

Characteristic	Type I (slow-twitch)	Type IIa (fast-twitch)	Type IIx (fast-twitch)
Motor neuron size	Small	Large	Large
Recruitment threshold	Low	Intermediate/High	High
Nerve conduction velocity	Slow	Fast	Fast
Contraction speed	Slow	Fast	Fast
Relaxation speed	Slow	Fast	Fast
Fatigue resistance	High	Intermediate/Low	Low
Endurance	High	Intermediate/Low	Low
Force production	Low	Intermediate	High
Power output	Low	Intermediate	High
Aerobic enzyme content	High	Intermediate	Low
Anaerobic enzyme content	Low	Intermediate/High	High
Sarcoplasmic reticulum complexity	Low	Intermediate/High	High
Capillary density	High	Intermediate	Low
Myoglobin content	High	Low	Low
Mitochondrial size/density	High	Intermediate/Low	Low
Fiber diameter	Small	Intermediate	Large
Color	Red	White/Red	White

This table summarizes the relative involvement of muscle fiber types in sport events:

Event	Type I	Type II
100 m sprint	Low	High
800 m run	High	High
Marathon	High	Low
Olympic weightlifting	Low	High
Soccer, lacrosse, hockey	High	High
American football wide receiver	Low	High
American football lineman	Low	High
Basketball, team handball	Low	High
Volleyball	Low	High
Baseball or softball pitcher	Low	High
Boxing	High	High
Wrestling	High	High
50 m swim	Low	High
Field events	Low	High
Cross-country skiing, biathlon	High	Low
Tennis	High	High
Downhill or slalom skiing	High	High
Speed skating	High	High
Track cycling	Low	High
Distance cycling	High	Low
Rowing	High	High

Motor unit recruitment

To increase force output, the nervous system activates additional motor units by recruiting more motor neurons.

• Key factors:
  • Small motor units (Type I fibers) are activated first for low-force tasks.
  • Larger motor units (Type II fibers) are recruited for high-force, high-speed tasks.
  • Effective training improves neural recruitment and coordination.

Proprioception

Muscle spindles

• Specialized sensory receptors located within muscles.
• Detect changes in muscle length and the rate of stretch.
• Activation triggers a reflex contraction that helps prevent overstretching.

Golgi tendon organs (GTOs)

• Located within tendons near the junction with muscle fibers.
• Monitor tension in the muscle-tendon unit.
• Activation inhibits further contraction to help protect muscles from excessive force.

How can athletes improve force production

• Use heavy loads to enhance neural recruitment
• Increase muscle cross-sectional area
• Perform explosive multijoint exercises to maximize fast-twitch fiber activation
• Apply periodized training to systematically target both neural adaptations (coordination, recruitment, and rate coding) and hypertrophic adaptations (increased muscle size) across different training phases